2.2.1 Cellulose

Cellulose, the major constituent of lignocellulosics, occupies up to 50% or more of the overall composition of the matter [1]. In general, it is considered to be the most abundant renewable material on the planet, with annual cellulosic biomass production in the order of approximately 2 teratons (or 2 × 10¹² tons). Although it has the same structural motif in each material that it comprises (i.e., the same corkscrew twofold symmetry), its degree of polymerization (DP or “n” shown in Figure 2.1) and crystallinity (degree of packing order as evidenced by X-ray crystallography) can vary widely. Estimates of the DP found in the literature show that it can vary from about n = 300 (in wood) to upward of 10,000 (cotton and bacterial cellulose, BC). It can also show wide trends in the overall crystalline morphology; for example, it can show a relatively low crystallinity in wood, whereas it can be highly crystalline in Valonia [2]. Nevertheless, cellulose tends to be rather nonreactive to chemicals other than cellulose enzymes. It is a linear polymer that is made up of β-1,4-linked d-glucopyranose units.

One of the glucose units within the chain can be described as having a rotation of 180° relative to its neighbor within the chain. The chemical description of a “dimer” or two units is cellobiose unit, which technically could be used to describe the polymeric structural pattern. Classically, a cellulose chain, in as much as its glucose monomer makeup, presents two different terminal hydroxyl groups: the non-reducing end group (NREG) and the reducing end group (REG). The NREG consists of a 4-hydroxyl group (at the 4 carbon), whereas the REG is categorized in organic chemistry as a hemiacetal linkage (or aldehyde hydrate group). Figure 2.2 shows a classical representation of the NREG and REG.

Each glucose unit presents three reactive hydroxyl groups (C₂, C₃, and C₆), which expedite the formation of both intra- and interchain hydrogen bonding. The stiffness and compactness of the chain can almost exclusively be attributed to interchain interactions. However, the structural uniformity of cellulose in any plant is lacking; typically, there exist both highly ordered (crystalline) domains and amorphous (low degree of order) domains, which as a rule depend on the raw material and the treatments it has been subjected to. The relative robustness of cellulose is critically dependent on these features.

Cellulose generally provides the main mechanical properties of any lignocellulosics owing to its packing and hydrogen-bonded structure; in general, it provides the overall load-bearing capacity of wood and plants but does not typically complex with the other biopolymers in the lignocellulosic matrix.

Cellulose is synthesized in vascular plants¹ in the plasma membrane of the rosette terminal complexes (RTCs), a class of macromolecular protein structures approximately 25 nm in diameter. These structures contain the cellulose synthase enzymes (at least three different sythases are involved that are encoded by CesA² genes) that are responsible for synthesizing the individual cellulose chains. The RTCs are able to spin a bundle of cellulose chains known as a microfibril into the cell wall. Scheme 2.1 shows a simplified representation of the production of microfibrils leading to cellulose fibers.

Native cellulose whose production is demonstrated in Scheme 2.1 is known from crystallographic data to have two distinct crystalline phases: Iα and Iβ [3]. Recently, exact representations of the principal phases of cellulose (Iα and Iβ) were obtained using atomic-resolution synchrotron and neutron diffraction data. The resulting structure of Iα was a one-chain triclinic unit cell (shown in Figure 2.3 with the principal axes: a, b, and c). The conformation of the glucosidic linkages and hydroxymethyl groups is identical, which is not the case for Iβ. In Iβ, there are two conformationally distinct chains in the monoclinic unit cell (corner and center chains) that requires adjacent glucosyl residues (in the same chain) to be the same. Both triclinic and monoclinic crystal systems derive from the seven crystal systems that are available to describe highly ordered systems.

The triclinic system can be best described as a crystal whose vector descriptors possess unequal length AND are not mutually orthogonal perpendicular to each other as in a classic three-dimensional Cartesian coordinate system. Similarly, a monoclinic system has vectors of unequal length, but they form a rectangular prism with a parallelogram – a base indicating that two vectors are orthogonal, but the third angle, γ, >90°.³

2.2.2 Heteropolysaccharides

Akin to cellulose in form, but not in function, are the heteropolysaccharides (or “hemicelluloses,” a more colloquial, but inexact term for this class of biomaterials) [4, 5]. This class of biomaterials is likely the second most dominant terrestrial material available after cellulose. In general, the amount of hemicelluloses is typically 20–30% of the dry weight of wood. They constitute a variegated class of materials without the chemical precision and homogeneity of cellulose, yet they are naturally produced annually in the order of 60 billion tons. This class of biomaterials is almost mandatorily associated with cellulose in any cellulosic matrix. They act in general to maintain the physical integrity of the cellulosic microfibrils and likely engage in a covalent complex (the lignin–carbohydrate complex, or LCC) with the lignin polymer of the plant cell [6]. They adopt a multiplicity of structural motifs, the most popular form being “xylan,” which is likely the third most abundant biomaterial on the planet [7]. Figure 2.4 shows several representations of xylan, a typical and globally dominant heteropolysaccharide.

As shown in Figure 2.4, heteropolysaccharides are very much unlike cellulose in a structural sense. Cellulose is crystalline, that is, well organized or packed, of a high molecular weight, and possesses a low polydispersity.⁴ However, heteropolysaccharides tend to be rather amorphous in their structure (often they are branched as shown in Figure 2.4), of a low molecular weight, and display a low PDI. In fact, the heteropolysaccharides typically do not have just a single repeating unit (such as xylan) but can often contain a multiplicity of monomers. For example, the dominant heteropolysaccharide is xylan, which tends to be fairly localized in angiosperms ( hardwoods), whereas the dominant heteropolysaccharide in gymnosperms (softwoods) is glucomannans. In addition, there are galactoglucomannans, arabinoxylans, and so on. The list is nearly endless in the natural diversity of heteropolysaccharides that are found. Figure 2.5 shows the structure, for example, of galactoglucomannan, a typical branched heteropolysaccharide.

Given the various physical characteristics stated here for heteropolysaccharides, it is no surprise that they tend to be more susceptible to degradation from both chemical and enzymatic attack. In addition, they can sometimes be more amenable to chemical derivatization and reaction than their cellulose analogue.

2.2.3 Lignin

This last polymer, albeit neglected for many years because of its molecular heterogeneity and seeming lack of tractability, is one of the most important polymers on the planet. It forms the last element of the lignocellulosics that primarily comprise the biomass on this planet. The term “lignin” is in fact taken from the Latin word lignum, which is translated to “wood.” It is found virtually in all biomass especially vascularized plants including herbs and grasses, but it is chiefly located (on a per capita basis) in the cell wall of woody tree species [8, 9]. On a mass basis, up to approximately 30% of all carbon in the biosphere can be attributed to lignin. A representation of its diverse native structure is provided in Figure 2.6 [10]. The structure seemingly lacks the signature monomer associated with polymer structures, but in wood chemistry circles, it is assumed that the monomer is based on a C₉ phenylpropanoid residue that will be further elaborated within this section.

The structure elucidated here is speculative at best in nature and represents a best guess attempt because not only is its X-ray structural characterization impossible, but it is extremely divergent in its structure between species and even within the same species. In terms of its role among the natural polymers in woody and plant species, lignin maintains unique and powerful functionality within the bio-system. It is essentially a natural “glue” that helps to keep the polysaccharide bundle intact and whole. The current theory for its particular niche in lignocellulosics is that it covalently complexes with the heteropolysaccharides in a so-called LCC. In addition to its chemical attachment to polysaccharides, it is believed that lignin also acts as a plant's second line of defense (after the bark/extractives) against microbial attack/infestation.

Lignin as shown in Figure 2.6 is a complex aromatic network polymer (but it is three-dimensional) that is polydisperse and contains a number of divergent functionalities and branching points. Although PDI may not be objectively applicable in the case of lignin, lignin nevertheless is a branched network polymer that has its origins in well-characterized lignol subunits. Figure 2.7 shows the principal monomeric units that constitute the biosynthesized lignin.

Each of the structures shown in Figure 2.7 is available in differing amounts in different classes of woods or plants. For example, monocotyledonous grasses tend to have almost exclusively the p-hydroxyphenyl monomer unit, whereas gymnosperms are almost exclusively constructed out of the guaiacyl units. Angiosperms, on the other hand, tend to have some combination of syringyl (principal component) and guaiacyl units.

The lignin extant in nature generally has a support role for the polysaccharide matrix. In woody tissue, it tends to surround the wood cell helping to ensure not only integrity but to maintain proper surface energetics (hydrophobicity) to allow for the uninterrupted circulation of water and nutrients. Figure 2.8 demonstrates the respective localization of lignin within a representative wood cell.

The cell wall is the region that contains the most lignin polymer, but it is highly concentrated between cells, in essence, to ensure good mechanical integrity among the macrofiber bundle [11]. This material has for roughly 130 years been the targeted polymer for deconstructing cellulosic fibers for papermaking. A huge global paper pulp industry has emerged since 1879 when Dahl, a German chemist, used sodium sulfate as a makeup chemical for soda pulping to regenerate sodium hydroxide. Serendipitously, sodium sulfide was formed and unexpectedly gave far better pulping results in terms of rates and pulp quality. The process was termed kraft from the German for “strong” [12].

2.2.4 The Discovery of Cellulose and Lignin

Anselme Payen (1795–1891) was born in Paris to Marc and Jean Payen, a family in which there was great respect for science, law, and chemistry. In fact, young Anselme (at the tender age of 13) began a lifelong love of studying science first with his father, a man whose entrepreneurial spirit led him to establish chemical factories, and then Anselme went off to study chemistry, physics, and mathematics at the École Polytechnique under the tutelage of Louis Nicolas Vauquelin and Michel Eugène Chevreul. Interestingly, after this quasi-internship, he returned to work for his father as the superintendent of a borax refining plant when he was only 23 years old (1818). What was so unique about this situation was that Payen's father had devised a better way of producing borax from boric acid that allowed him better market opportunities. After Payen's father died in 1820, he held sole custody of the family estate. Payen turned his interest to one of his father's factories that refined sugar from beets. Interestingly, he employed activated charcoal to decolorize the sugar, a method that has been in vogue ever since that impressive discovery. In addition, this work helped to expedite the transition, on a world stage, from obtaining sugar from cane to beets. One of his most remarkable efforts and perhaps what he may be best known for is the discovery of diastase (from the Greek for “separate”), an enzyme that converts starch to glucose. He was able to isolate this substance from malt extract in 1833; interestingly, it was the first isolated enzyme, an organic system that demonstrates catalysis, that is, enhancing the rate of reaction without being consumed. His choice of terminology, diastase, led to the tradition of using the suffix -ase in biochemistry for the naming of enzymes. In 1834, Payen began the systematic study of wood from which he discovered a substance from the plant cell walls that could be hydrolyzed (broken down with water) to glucose units (similar to starch). He called the substance “cellulose,” which began another tradition of using “-ose” as the suffix to name carbohydrates. In 1835, he departed from industrial work in favor of a professorship of industrial and agricultural chemistry at the École Centrale des Arts et Manufactures. In 1839, he then accepted a joint appointment at the nearby Conservatoire des Arts, which he held in conjunction with his original appointment until his death in 1871. He was a prolific researcher, publishing over 200 papers and 10 books on topics such as dextrans, sugars, lignin, cellulose, and starch. His seminal paper on cellulose and lignin isolation was published in 1838. These experiments in 1838 revealed that in addition to cellulose, wood contained an oxidizable crusty substance that was later designated as “lignin” in 1857 by Schulze. Payen treated the wood with a concentrated nitric acid solution and later washed the residue with an alkaline solution (sodium hydroxide) to dissolve the crusty substance. Payen noted significant differences between the wood and the crusty substance; he is also credited with the first Klason lignin isolation method whereby he used concentrated sulfuric acid to remove the polysaccharides, which then allowed for the isolation of the “lignin” fraction. Today, the American Chemical Society (ACS) honors Anselme Payen's memory by awarding each year a prize in his name to the scientist who in the opinion of the Cellulose and Renewable Materials Division (CELL) of the ACS has contributed the most to the science, engineering, and technology of cellulose and renewable materials (more information can be found at the ACS site: http://cell.sites.acs.org/anselmepayenaward.htm).

2.3.1 Cellulose Reactivity

One of the complicating factors in the proper understanding of the reactivity of cellulose is its accessibility in addition to its chemical makeup. It is organized elegantly within a lignocellulosic matrix that consists of its interplay with the heteropolysaccharides and the lignin. Today, the reactivity of cellulose is a topic of grave importance because of its ability to supply an ever burgeoning bio-fuels and biomaterials community [13, 14]. As demonstrated earlier, the reactivity of cellulose is a function of accessibility, which is severely hampered by its compact structure. This compact structure is a function of the presence of a very strong hydrogen bonding network that gives rise to highly ordered region.

Critical factors that have been suggested by a number of experts include the number and size of pores in the cellulose structure, the molecular size and type of chemical that is added to the cellulose, the internal surface as controlled by the size of fibrils/aggregates, and the morphology of the cellulose macromolecules. These critical factors can only be addressed by (i) ensuring that the cellulosic pores are sufficiently open to accommodate the chemicals/reagents added, (ii) deconstructing fibrillar aggregates, and (iii) deconstructing the ordered regions to no longer adopt a stiff, compact network structure, that is, the hydrogen bonding network must be disrupted. An optimal chemical interaction, therefore, must consider these latter three criteria; recently, chemical, mechanical, and biological treatments have been tested, with a great push toward the environmentally benign biological treatments.

Chemical treatments of cellulose [15] are employed to enhance the swelling of the cellulose fibers. This swelling not only facilitates the passage of chemical agents, but its primary purpose is to disrupt the hydrogen bonds because of the high osmotic pressure induced by the swelling phenomenon. Thus, the hydrogen-bonded network becomes disrupted, and as a result the compact structure is no longer available leading to a more accessible structure. A pictorial representation of this phenomenon is shown in Figure 2.9.

The scheme illustrates the inclusion of water molecules that interfere with the hydrogen bonding between the C₆, and the C₃ and C₂ hydroxyls of chain neighbors and act to therefore swell the cellulosic substrate. As a result of this inclusion of water molecules, the swelled structure loses its ordered nature and in some cases, there is a complete loss of crystallinity, which consequently leads to an increase in the active surface area or exposed hydroxyl groups (which were heretofore buried within the tightly packed cellulosic crystallite). There have been a number of studies within this particular area that have shown such accessibility to solvents including among others the use of sodium hydroxide and its combination with urea. Urea is conjectured to insert within the interchain cellulosic matrix and disrupt the hydrogen bonding paradigm. The purpose of such treatments is to facilitate accessibility for a particular

In addition to chemical treatments, cellulosics can be treated with a mechanical process [16]. Mechanical treatment of the cellulose fibers is used in the pulp and paper industry because of its capacity to enhance fiber–fiber bonding, to cut or make the fibers stronger, and to produce changes on the cellulose structure. For instance, strong bonds among fibers give the printing paper strong and smooth properties. When the pulp is subjected to mechanical treatment, the interfibrillar bonds, which are mainly located in the primary wall and in the outer lamella of the S1 layer of the cell wall, are disrupted. This effect leads to an increase in the reactive surface area of the fibers, improving the accessibility of the cellulose. In several studies, mechanical treatment has been used in combination with other treatments.

Moreover, enzymatic treatments can be used as well [17]. Enzymes have broad industrial applications. They have been used in the detergent, food, and pharmaceutical sectors. Enzymes have also been studied in the pulp and paper industry, and they are currently used for several applications, including deinking and as bleaching agents. The effect of enzymatic treatments on cellulose reactivity has also been investigated. It has been reported that enzymatic treatments, especially that of cellulases on dissolving pulps, hold a great potential for increasing cellulose reactivity.

One of the enzymes is the cellulases: monocomponent endoglucanases. Cellulases are enzymes that hydrolyze the 1,4-β-d-glucosidic bonds of the cellulose chain. There are three major groups of cellulases: endoglucanases, cellobiohydrolases or exoglucanases, and glucosidases. These enzymes can act alone or together on the cellulose chain or together. When they act together, a synergistic phenomenon is often generated, resulting in an efficient degradation of the cellulose structure.

Endoglucanases are enzymes that randomly cleave the amorphous sites of the cellulose creating shorter chains (oligosaccharides) and, therefore, new chain ends. Cellobiohydrolases or exoglucanases attack the reducing and non-reducing ends of the cellulose chains, generating mainly glucose or cellobiose units. This type of cellulose can also act on microcrystalline cellulose by a peeling mechanism. Glucosidases act on cellobiose generating glucose units. It has been suggested that there are three primary parameters affecting the degree of enzymatic hydrolysis: the crystallinity, the specific surface area, and the degree of polymerization of the cellulose.

Most cellulases consist of two domains. The first is a catalytic domain, which is responsible for the hydrolysis of the cellulose chain. The catalytic domain of endoglucanases is “cleft shaped.” Exoglucanase, on the other hand, has a “tunnel-shaped” catalytic domain structure. The second is a cellulose-binding domain (CBD), which helps the enzyme to bind to the cellulose chain bringing the catalytic domain close to the substrate. An interdomain linker serves as a connection between the two domains.

2.3.2 Hemicellulose Reactivity

Several methods have been used to extract hemicellulose from woody tissues [26–28]. Those methods include dilute-acid pretreatments, alkaline extraction, alkaline peroxide extraction, liquid hot-water extraction, steam treatment, microwave treatment, and ionic liquid extraction.

Dilute-acid pretreatment, often involving about 0.5–1% sulfuric acid, is a useful procedure for hemicellulose isolation. By using this method, the majority of the original amount of hemicellulose from the poplar tree (hardwood) can be recovered as dissolved sugar. However, in the course of such treatment, a high amount of the hemicellulose monomers are degraded, leading to the generation of by-products. On the other hand, extraction of hemicellulose from sugarcane bagasse by using hot-water extraction at a temperature of 150–170 °C recovers almost 90% of the hemicellulose as dissolved sugar, but with relatively less monomer degradation than that of the dilute-acid method. Because hot-water treatment cleaves some of the acetate groups from hemicellulose, the pH decreases. The reduced pH results in additional generation of acetic acid, leading to a phenomenon called “autohydrolysis.” It has been shown that autohydrolysis can be promoted by irradiating wood with microwaves in water.

Due to severe treatments caused by acidic and partly applied heated extraction processes, hemicellulose degrades by losing its chain length, and consequently, exhibits high polydispersity. Steam and microwave treatments are combined with chemicals to dissolve the hemicellulose; however, due to the complexity of those methods, they have been applied mainly on a small scale for hemicellulose extraction. In steam treatment, ester bonds in the hemicelluloses will be cleaved and then the wood is treated with steam, resulting in the formation of acetic acid. This will lower the pH and, thus, induce autohydrolysis of the glycosidic bonds in the hemicelluloses. Such a process will generate a low-molecular-weight, water-soluble hemicellulose. From this method, one can get a low percentage yield of hemicellulose with significant contamination of dissolved cellulose and lignin and degradation products of the hemicellulose.

An ionic liquid/cosolvent extraction system has been used by Froschauer and coworkers to separate hemicellulose and cellulose from wood pulp. In this study, hemicellulose-rich birch kraft pulp was selectively separated, with high levels of purity, into pure cellulose and hemicellulose fractions by using mixtures of cosolvents (water, ethanol, or acetone) and the cellulose-dissolving ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIM OAc) with reaction conditions of 60 °C for 3 h under stirring. This process was used to generate dissolving pulp that met the manufacture of revived cellulose products and cellulose derivatives because of its extraordinary cellulose content, which was assumed to be more than 90%, as well as the product's high brightness and uniform molecular-weight distribution.

Alkaline extraction is well studied and is known as a strong and efficient method for hemicellulose extraction. Indeed, alkaline treatment can cleave the ester linkage between ferulic acid of lignin and the glucan and arabinan residues of hemicellulose in the cell wall, thus releasing oligo- and polymeric hemicellulose rather than sugars, as obtained from dilute-acid extraction or hot-water extraction. The extraction of hemicelluloses from various biomasses is expected to be closely related to the amount of LCCs. The LCCs entail covalent linkages between lignin and carbohydrates, mainly hemicelluloses. The major types of LCCs include phenyl glycoside, benzyl ether, and benzyl ester types of linkages. Different biomasses have different frequencies of LCC, for example, the amounts of ether and ester LCC linkages in pine and aspen cellulolytic enzyme lignin (CEL) have been reported to be about 2.2–2.5 and 0.3–0.6 per 100 monomeric lignin units, respectively.

The decision on which method to use for hemicellulose extraction is highly dependent on the final application of the recovered hemicellulose. For example, if the extracted hemicellulose is targeted for the production of bio-ethanol, then the monomeric form of sugars is required, and thus, it may be preferred to use dilute acid or hot water. Some other applications require a high-molecular-weight polymer (blended plastics, hydrogels, and others). For those applications, it may be extremely important to use the alkaline method for the isolation of hemicellulose from the biomass.

2.3.3 Lignin Reactivity

Many new techniques were developed to isolate lignin formulations to study [29–33]. Generally, three approaches were taken toward investigating lignin structure: degradation reactions, biosynthetic work, and spectroscopy studies. In the most common approach, lignin formulations were subjected to various degradation reactions yielding identifiable products that gave useful structural information. Many new analytical techniques were developed during these times that were highly useful for the lignin chemists. In the final approach, model compounds were synthesized from postulated lignin precursors to produce new products for study. This was a time in which much work was applied to developing new methods to isolate lignin. In 1936, Bailey at the University of Washington showed by microdissection that the middle lamella was composed mainly (72%) of lignin rather than pectin. In 1935, Van Beckum and Ritter at the US Forest Products Laboratory (USFPL) removed lignin from plant tissues with hypochlorite followed by NaOH. The material that remained termed holocellulose consisted of the total carbohydrate mass present in the plant tissues. In 1939, Brauns reported that neutral solvent extraction of woody tissue and subsequent purification created a few percent of what he named native lignin or Braun's lignin. Presently, many investigators view this material as a mixture of lower molecular weight lignins and/or lignans. This work reflected the continuing search by some classical organic chemists for a lignin that could be extracted simply by use of solvents without chemical reaction. In 1947, Richie and Purves at McGill University oxidized wood at pH 4 with aqueous 5% sodium periodate. The periodate lignin preparation thus obtained was 86–96% Klason lignin, insoluble in organic solvents even at boiling temperature but completely soluble under conditions of sulfite pulping. A periodate lignin from spruce closely duplicated the behavior of spruce lignin in situ toward many degradative procedures.

2.4.1 Rationale and Significance

Plastics industries manufacture has a wide variety of materials, which plastics are produced or derived from major non renewable energy sources such as crude oil, natural gas and coal. Nearly 6% of the world's crude oil production is used for making approximately 245 million metric tons of plastics globally on an annual basis. These are used to meet both the requirements of cheap mass production and of highly specific applications. The worldwide economy dependence on petroleum-based plastics is not sustainable, based on the extremely volatile oil and energy situation, coupled with major changes in supply and demand patterns. The fluctuation in costs is a challenge but in addition, the limited feedstock availability is tightening and impacting supply and demand worldwide, and putting the industry under tremendous pressure.

Moreover, plastics now make up a significant part of a typical municipal solid waste (MSW) stream, and represent the fastest growing component. A huge 44 billion pounds of plastics enter United States' MSW stream each year, equivalent to ½ pound per day per person. In the United States, plastics on average account for 10% by weight of MSW, more than metals (8%) and glass (6%). The cost of waste management is now a matter of great public concern. There is more carbohydrate on earth than all other organic material combined. Polysaccharides are the most abundant type of carbohydrate and make up approximately 75% of all organic matter. The use of biodegradable, starch- and wood-based products as proposed here would be a significant improvement to the economy, environment, and society. Furthermore, starch-cellulosic fibers are viable feedstocks for enzymatic conversion to ethanol or are high heat value material suitable for combustion, alternatives to landfilling.

Many industry announcements regarding new and innovative plastic products occur on an ever more frequent basis. Coca-Cola recently announced that they will begin utilizing polyethylene terephthalate (PET) bottles containing 30% renewable content from sugarcane-derived ethylene glycol. They also announced plans to convert all their plastic packaging to the new material by 2020. Heinz will use the same material to make 120 million bottles for their ketchup products this year. PepsiCo claims to have developed the world's first totally bio-based PET bottle. It is made from biomass including switchgrass, pine bark, and corn husks. Pilot-scale production began in 2012. Other interesting new materials entering the market include a new family of resins (Panacea) containing 10–40% finely ground soy-based protein and an injection mold-grade cellulose-based resin. The cellulose-based resin is being used to make the first biodegradable tubes for toothpaste.

Much of the focus on renewable and sustainable plastics involves the use of starch either as a feedstock or as a component for industrial products. Industrial products in the United States that utilize starch have grown from 13 million metric tons (MMT) in 1975 to over 160 MMT today. Starch is inexpensive, widely available, and one of the most abundant biomass products in nature. It is produced in many different plant organs including roots, leaves, seeds, and stems. However, the hydrophilic nature of starch and its tendency to embrittle with age do not make it suitable as a replacement for plastics – hence, the importance of adding lignocellulose to the starch matrix system.

2.4.2 Starch-Based Materials

The development and production of biodegradable starch-based materials have been spurred by oil shortages and the growing interest in easing the environmental burden of petrochemically derived polymers. Starch is one of the most studied and promising raw materials for the production of biodegradable plastics, which is a natural renewable carbohydrate polymer obtained from a great variety of crops (Figure 2.10). Starch is low cost material in comparison to most synthetic plastics and is readily available. Starch has been investigated widely for the potential manufacture of products such as water-soluble pouches for detergents and insecticides, flushable liners and bags, and medical delivery systems and devices. Native starch commonly exists in a granular structure, which can be processed into thermoplastic starch (TPS) under the action of high temperature and shear by melt extrusion. Unfortunately, the properties of TPSs are not satisfactory for some applications such as packaging materials and composites engineering products.

One of the unique characteristics of starch-based polymers is their processing properties, which are much more complex than conventional polymers. The processing of starch-based polymers involves multiple reactions, for example, water diffusion, granule expansion, gelatinization, decomposition, melting, and crystallization (Figure 2.11). Among the various phase transitions, gelatinization is particularly important because it is closely related to others, and it is the basis for the conversion of starch to a thermoplastic. Furthermore, the decomposition temperature of starch is higher than its melting temperature before gelatinization. Without physical force (shear stress), the process of gelatinization depends mainly on water content and temperature conditions. A previous study has shown that shear stress can result in the fragmentation of starch granules during extrusion. Indeed, both the mechanical and thermal energies are transferred to starch dough during extrusion in molten medium. The main objectives of most starch-processing techniques are melting and mixing, which are adjusted to minimize chain degradation.

Starch use in papermaking dates back to the invention of paper 2000 years ago, when it was applied to obtain a stronger and smoother writing surface. Starch contributes to paper manufacturing because it serves as a binding agent that can enhance the mechanical properties of paper and improve paper manufacturing by increasing paper pulp retention on the paper machine. Starch was also chemically modified into cationic forms to further improve interconnections between fibers, increasing thus the paper strength. Non-covalent binding of starch and cellulosic fiber in biocomposites were created using a polymer suspension either by, for example, drying or hot-pressing.

2.4.3 Starch-Based Plastics

Plastic ranks as the second most used packaging material in the United States. In contrast to paper, only 7% of plastic generated as waste is recycled. This explains why more plastics ultimately end up in landfills than paper or any other packaging material. China, one of the world's largest plastic producers, noted that the largest source of its marine pollution was from discharging wastewater to sea. In Europe alone, an estimated 2–3 million tons of plastics is used each year in agricultural applications. Polyethylene films are used extensively to increase yields, extend growing seasons, reduce the usage of pesticides and herbicides, and help conserve water. Films made of starch blends were some of the first films containing renewable content tested as agricultural mulch.

Packaging and containers make up the largest sector (29.5%) of plastic waste in MSW. Plastic packaging has become an integral part of the global marketplace. Today, packaging design has begun integrating sustainability as never before, in part, because sustainability itself has become a marketing angle. Retailers are now realizing that customers respond positively to products marketed in more sustainable or green packaging.

The interest in utilizing starch as a replacement for plastics started in the 1970s and intensified in the 1980s right along with the dramatic growth in the use of plastics worldwide and the concerns about the effects of plastics on the environment (Figure 2.12)

But starch by itself is a poor substitute for petroleum plastics due to its moisture sensitivity and inferior mechanical properties. However, numerous strategies have been tested resulting in commercialized technologies for incorporating starch in plastics. A survey of the more common approaches has been included in this section as follows: